1. Field of the Invention
The present invention pertains to a method for controlling fugitive dust emission from granular (particulate) fertilizers. The present invention is specifically directed to the use of an aqueous solution of a mixture of lignosulfonate and urea as a fugitive dust suppressant for granular fertilizers.
2. Description of Related Art
Fugitive dust emission from granular fertilizers has, in recent years, been an increasing concern because of the growing reliance on bulk handling of fertilizers, in preference to bags, and because of the heightened awareness of the potential health hazards of airborn dust. Dustiness, in large part, is due to inefficient removal of fines during fertilizer manufacture, poor granule strength, abrasion of fine surface crystals, and poorly adherent anticaking additives. As a consequence, a substantial amount of dust is created during handling and transportation of granular fertilizers.
Techniques are known for reducing the dustiness of prilled urea fertilizer. For example, it is known to add a small amount of formaldehyde to urea during urea manufacture to increase hardness. although safety and health concerns have limited this approach. Urea also has been treated with a small amount of lignosulfonate to increase its hardness, see U.S. Pat. No. 4,578,358. Numerous attempts to reduce dustiness of inorganic fertilizers, however, through improvements in the fertilizer manufacturing process, have been largely unsuccessful.
The benefits of lignosulfonate as a granulation aid have long been known in the fertilizer art. U.S. Pat. No. 3,725,029 discloses using a concentrated lignosulfonate solution as a binder for granulating ammonium sulfate. Recent U.S. Pat. No. 4,846,871 claims that the caking tendency and dustiness of inorganic fertilizers, including phosphates, nitrates and the like can be reduced by adding lignosulfonate during the granulation procedure to distribute it homogeneously throughout the fertilizer granules. Russian Patent 1,096,265 describes using an aqueous lignosulfonate-urea solution as a binder to consolidate a potassium chloride powder (dust) fraction and fine grain potassium chloride, assisted by mechanical pressing.
One of the most commonly used procedures for fertilizer dust control, especially for inorganic fertilizers, has been simply to spray the fertilizer with a small amount, generally about 0.5% by weight, of a petroleum oil to bind the dust to the granules. This treatment is only short-lived, because of evaporative loss of the oil, and cannot be used at all with high nitrate fertilizers (e.g., ammonium nitrate) because it creates an explosion hazard. Other liquids also have been used for fertilizer dust control including lignosulfonate solutions, amines, surfactants, waxes, wax emulsions and water alone, but have not proved completely satisfactory. One prevalent problem is that the use of aqueous treatment solutions tends to exacerbate the caking problem often encountered with inorganic fertilizers. Thus, there remains a need in the art for a treatment which effectively reduces the level of fugitive dust emission from inorganic fertilizers.